Refrigeration system for transcritical operation with economizer and low-pressure receiver

ABSTRACT

A refrigeration apparatus for transcritical operation includes a gas cooler, an aftercooler, an evaporator with a low-pressure liquid separator, a compressor, a first controllable throttling device and interconnecting piping between the mentioned components and in addition a second controllable throttling device and an intercooler that comprises two flow paths separated by heat exchanging surfaces. A first flow path inlet of the intercooler is connected to the gas cooler outlet. A first flow path outlet of the intercooler is connected to the aftercooler inlet. A second flow path inlet of the intercooler is connected to the outlet of the second throttling device. A second flow path outlet of the intercooler is connected to the economizer port of the compressor. The second throttling device inlet is connected to the piping either upstream or downstream of the aftercooler. The second throttling device outlet is connected to the second flow path inlet of the intercooler.

This invention relates to a refrigeration apparatus for transcritical operation with screw compressors featuring geometrically controlled inlet and outlet ports operating at least on three pressure levels. The pressure levels comprise the suction pressure prevailing on the compressor suction side and being close to the pressure in the evaporator, the intermediate pressure prevailing at the economizer port, and the discharge pressure acting on the compressor discharge side and being close to the pressure in a gas cooler. The pertinent sides of the compressor are also designated as low-pressure side, intake side or suction side, and as high-pressure side or discharge side respectively. The pressure on the high-pressure side is higher than the pressure at the critical point of the refrigerant. Therefore, this process is designated as transcritical or overcritical refrigeration process. The economizer port is arranged between suction- and discharge side of the compressor. At the economizer port, the inlet process to the working cavity starts when there is no more flow connection of this working cavity to the compressor suction side. In this phase, the geometric volume of the working cavity considered has reached its maximum. Depending on the wrap angle of the rotor profile of the male rotor, number of lobes of both rotors, the geometric volume of the working cavity considered can be constant (transfer phase) or can decrease due to rotation of rotors.

The invention relates to a refrigeration apparatus featuring a heat exchanger, a so-called aftercooler, arranged in or at the low-pressure liquid separator and communicating with the liquid separator, and in this aftercooler the refrigerant—the working fluid—being under discharge pressure is subcooled prior to its expansion nearly to evaporation temperature, thus changing from the vaporous phase to the liquid phase, before it is expanded into the evaporators at the throttling device of the refrigeration apparatus.

The pressure upstream of this throttling device is kept constant by opening or closing it more or less respectively enabling the compressor to operate at constant discharge pressure. The refrigerating capacity of the refrigeration apparatus changes depending on the temperature to which the refrigerant was cooled down in the gas cooler. It will be reduced as a result of higher outlet temperatures at the gas cooler, because at higher gas cooler outlet temperatures more working fluid will evaporate in the low-pressure liquid separator for cooling-down the working fluid in the aftercooler prior to expansion than at lower gas cooler outlet temperatures. Therefore, the efficiency of the refrigeration apparatus will decrease with increasing temperature at the gas cooler.

The object of the invention is to improve the process and to increase the efficiency of the refrigeration apparatus.

According to the invention the refrigeration apparatus for transcritical operation comprises in addition to the components gas cooler, aftercooler, evaporator with low-pressure liquid separator, compressor, first controllable throttling device and interconnecting piping between the mentioned components a second controllable throttling device and an intercooler that comprises two flow paths separated by heat-exchanging surfaces, wherein a first flow path inlet of the intercooler is connected to the gas cooler outlet, a first flow path outlet of the intercooler is connected to the aftercooler inlet, a second flow pass inlet of the intercooler is connected to the outlet of the second throttling device and a second flow pass outlet of the intercooler is connected to the economizer port of the compressor, and the second throttling device inlet is connected to the piping either upstream or downstream of the aftercooler and the second throttling device outlet is connected to the second flow pass inlet of the intercooler.

According to the invention, a part of the refrigerant is taken from the main flow either upstream or downstream of the aftercooler and led via the second controllable throttling device, where the refrigerant pressure decreases from discharge pressure to intermediate pressure and the temperature drops, to the second flow path of the intercooler to cool down the working fluid in the first flow path of the intercooler. In this way, the refrigerant being under discharge pressure is cooled down on one side of the heat-exchanging surfaces of the intercooler, while the refrigerant on the other side of the heat-exchanging surfaces of the intercooler evaporates being under intermediate pressure. The refrigerant evaporated is led to the economizer port of the compressor.

Due to this operation of the intercooler, the aftercooler is unloaded. As a result of the unloading, less amount of vapor is created in the aftercooler on the side of the low-pressure liquid separator. Thus, with the same compressor size, more vapor can be taken from the evaporator. Therefore, the refrigerating capacity of the refrigeration apparatus and its efficiency will increase.

In the following, the invention is explained in detail by an example of embodiment.

The accompanying drawings show in:

FIG. 1 a simplified schematic for arrangement of compressor and heat exchangers with pertinent interconnecting piping and control devices of the refrigeration apparatus according to the invention.

FIG. 2 a Pressure-Enthalpy diagram for a refrigeration- or air conditioning apparatus according to the invention.

FIG. 3 a simplified schematic for arrangement of compressor and heat exchangers with pertinent interconnecting piping and control devices for another arrangement example of a refrigeration apparatus according to the invention.

FIG. 4 a Pressure-Enthalpy diagram for the arrangement according to the invention in compliance with FIG. 3.

The refrigeration apparatus for transcritical operation according to FIG. 1 comprises a gas cooler 23, an intercooler 24, an evaporator 30, a low-pressure liquid separator 25 communicating with an aftercooler 27, a screw compressor 21 having geometrically controlled inlet and outlet ports, a first controllable throttling device 28, a second controllable throttling device 26 and interconnecting piping between the components mentioned. When compressor 21 is in operation, suction pressure 11 prevails on its suction side 29, while discharge pressure 12 prevails on its discharge side 22 with the pressure on the discharge side 22 being higher than the pressure at the critical point of the refrigerant. The compressor has an economizer port 31 at the housing enabling a flow connection to intercooler 24, and the pressure in this pipe section lies between discharge pressure and suction pressure.

In the Pressure-Enthalpy diagram according to FIG. 2, point 1 describes the condition on the suction side of compressor 21. The outlet condition of the refrigerant after compressor 21, point 2, is the inlet condition into gas cooler 23. The refrigerant passes gas cooler 23 which is fed by a cooling medium, e.g. cooling water, for cooling the refrigerant vapor. When leaving said gas cooler 23, the refrigerant has the condition at point 3. In intercooler 24 through which two refrigerant flows of the refrigeration apparatus are led, the refrigerant is cooled from point 3 to point 4. For this purpose, the partial refrigerant flow expanded to intermediate pressure level 10 will be evaporated and superfed via economizer port 31 into the compressor without considerably influencing the suction volume flow. The refrigerant flow is further cooled from point 4 to point 5 in aftercooler 27 wherein liquid evaporates in aftercooler 27 communicating with low-pressure liquid separator 25, and hence reducing the available volumetric refrigerating capacity by the enthalpy difference from point 1 to point 9. Point 9 corresponds to the condition of the refrigerant at the evaporator outlet 35 characterized by a two-phase mixture. The intermediate pressure level 10 can be used for changing the refrigerating capacity by way of rising the intermediate pressure, and hence changing the intermediate cooling effect.

Due to cooling the refrigerant vapor in intercooler 24, there will be created less vapor in aftercooler 27 on the side of low-pressure liquid separator 25. Thus, with the same compressor size, more vapor can be taken from the evaporator. Therefore, the refrigerating capacity of the refrigeration apparatus and its efficiency will increase.

The refrigeration apparatus for transcritical operation according to FIG. 3 is configured similarly to FIG. 1 with the distinguishing feature that the second flow path of intercooler 24 on its inlet side is connected via piping and second controllable throttling device 32 to the outlet of an intermediate-pressure aftercooler 34. The inlet of the intermediate-pressure aftercooler 34 is connected to the outlet of the first flow path of intercooler 24. The outlet side of the second flow path of intercooler 24 is connected to economizer port 31 of compressor 21 via an intermediate-pressure liquid separator 33. Intermediate-pressure aftercooler 34 communicates with intermediate-pressure liquid separator 33.

In the Pressure-Enthalpy diagram according to FIG. 4, point 4′ describes the outlet condition from intermediate-pressure aftercooler 34, point 13 describes the inlet condition into intercooler 24 and point 17 describes the outlet condition from intercooler 24.

LIST OF REFERENCE NUMERALS USED

-   1. Point -   2. Point -   3. Point -   4. Point -   4′. Point -   5. Point -   9. Point -   10. Intermediate-pressure level -   11. Suction pressure -   12. Discharge pressure -   13. Point -   17. Point -   21. Screw compressor -   22. Compressor discharge side -   23. Gas cooler -   24. Intercooler -   25. Low-pressure liquid separator -   26. Second controllable throttling device -   27. Aftercooler -   28. First controllable throttling device -   29. Compressor suction side -   30. Evaporator -   31. Economizer port -   32. Second controllable throttling device -   33. Intermediate-pressure liquid separator -   34. Intermediate-pressure aftercooler -   35. Evaporator outlet -   36. First flow path -   37. Second flow path -   38. First flow path inlet/Gas cooler outlet -   39. First flow path outlet -   40. Second flow path inlet -   41. Second flow path outlet -   42. Inlet of the second controllable throttling device -   43. Outlet of the second controllable throttling device -   44. Aftercooler inlet -   45. Piping 

1. A refrigeration apparatus for transcritical operation comprising a screw compressor featuring inlet and outlet ports geometrically controlled by rotation of male and female rotors operating at least on three pressure levels: suction pressure on the compressor suction side, intermediate pressure at the economizer port and discharge pressure at the compressor discharge side, a gas cooler, a low-pressure liquid separator, an aftercooler communicating with said low-pressure liquid separator, a first controllable throttling device and an evaporator, wherein there is a second controllable throttling device and an intercooler that comprises a first and a second flow path separated by heat-exchanging surfaces.
 2. A refrigeration apparatus according to claim 1 wherein a first flow path inlet of said intercooler is connected to the gas cooler outlet, a first flow path outlet of said intercooler is connected to the aftercooler inlet, a second flow pass inlet of said intercooler is connected to the outlet of said second controllable throttling device and a second flow pass outlet of said intercooler is connected to the economizer port of the compressor.
 3. A refrigeration apparatus according to claim 1 wherein the inlet of said second controllable throttling device is connected to the piping either upstream or downstream of the aftercooler and the outlet of said second controllable throttling device is connected to said second flow pass inlet of said intercooler. 